One last concept required for our discussion is propensity score. Introduced by [1]} it is defined to be the probability of treatment assignment conditioned on observed covariates. Mathematically it can be described as \(e = P(T \mid X)\)  where \(X\)  are the covariates and \(T\)  the treatment. Commonly used as a matching criterion to form sets of treated and untreated subjects that are close in the covariate space; these sets are subsequently used to estimate the causal effect of the treatment.
